reftable/record: introduce function to compare records by key

In some places we need to sort reftable records by their keys to
determine their ordering. This is done by first formatting the keys into
a `struct strbuf` and then using `strbuf_cmp()` to compare them. This
logic is needlessly roundabout and can end up costing quite a bit of CPU
cycles, both due to the allocation and formatting logic.

Introduce a new `reftable_record_cmp()` function that knows how to
compare two records with each other without requiring allocations.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 68 insertions, 1 deletions

diff --git a/reftable/record.c b/reftable/record.c
index 5c3fbb7b2a..f1b6a5eac9 100644
--- a/reftable/record.c
+++ b/reftable/record.c
@@ -430,7 +430,6 @@ static int reftable_ref_record_is_deletion_void(const void *p)
 		(const struct reftable_ref_record *)p);
 }
 
-
 static int reftable_ref_record_equal_void(const void *a,
 					  const void *b, int hash_size)
 {
@@ -439,6 +438,13 @@ static int reftable_ref_record_equal_void(const void *a,
 	return reftable_ref_record_equal(ra, rb, hash_size);
 }
 
+static int reftable_ref_record_cmp_void(const void *_a, const void *_b)
+{
+	const struct reftable_ref_record *a = _a;
+	const struct reftable_ref_record *b = _b;
+	return strcmp(a->refname, b->refname);
+}
+
 static void reftable_ref_record_print_void(const void *rec,
 					   int hash_size)
 {
@@ -455,6 +461,7 @@ static struct reftable_record_vtable reftable_ref_record_vtable = {
 	.release = &reftable_ref_record_release_void,
 	.is_deletion = &reftable_ref_record_is_deletion_void,
 	.equal = &reftable_ref_record_equal_void,
+	.cmp = &reftable_ref_record_cmp_void,
 	.print = &reftable_ref_record_print_void,
 };
 
@@ -625,6 +632,25 @@ static int reftable_obj_record_equal_void(const void *a, const void *b, int hash
 	return 1;
 }
 
+static int reftable_obj_record_cmp_void(const void *_a, const void *_b)
+{
+	const struct reftable_obj_record *a = _a;
+	const struct reftable_obj_record *b = _b;
+	int cmp;
+
+	cmp = memcmp(a->hash_prefix, b->hash_prefix,
+		     a->hash_prefix_len > b->hash_prefix_len ?
+		     a->hash_prefix_len : b->hash_prefix_len);
+	if (cmp)
+		return cmp;
+
+	/*
+	 * When the prefix is the same then the object record that is longer is
+	 * considered to be bigger.
+	 */
+	return a->hash_prefix_len - b->hash_prefix_len;
+}
+
 static struct reftable_record_vtable reftable_obj_record_vtable = {
 	.key = &reftable_obj_record_key,
 	.type = BLOCK_TYPE_OBJ,
@@ -635,6 +661,7 @@ static struct reftable_record_vtable reftable_obj_record_vtable = {
 	.release = &reftable_obj_record_release,
 	.is_deletion = &not_a_deletion,
 	.equal = &reftable_obj_record_equal_void,
+	.cmp = &reftable_obj_record_cmp_void,
 	.print = &reftable_obj_record_print,
 };
 
@@ -953,6 +980,22 @@ static int reftable_log_record_equal_void(const void *a,
 					 hash_size);
 }
 
+static int reftable_log_record_cmp_void(const void *_a, const void *_b)
+{
+	const struct reftable_log_record *a = _a;
+	const struct reftable_log_record *b = _b;
+	int cmp = strcmp(a->refname, b->refname);
+	if (cmp)
+		return cmp;
+
+	/*
+	 * Note that the comparison here is reversed. This is because the
+	 * update index is reversed when comparing keys. For reference, see how
+	 * we handle this in reftable_log_record_key()`.
+	 */
+	return b->update_index - a->update_index;
+}
+
 int reftable_log_record_equal(const struct reftable_log_record *a,
 			      const struct reftable_log_record *b, int hash_size)
 {
@@ -1002,6 +1045,7 @@ static struct reftable_record_vtable reftable_log_record_vtable = {
 	.release = &reftable_log_record_release_void,
 	.is_deletion = &reftable_log_record_is_deletion_void,
 	.equal = &reftable_log_record_equal_void,
+	.cmp = &reftable_log_record_cmp_void,
 	.print = &reftable_log_record_print_void,
 };
 
@@ -1077,6 +1121,13 @@ static int reftable_index_record_equal(const void *a, const void *b, int hash_si
 	return ia->offset == ib->offset && !strbuf_cmp(&ia->last_key, &ib->last_key);
 }
 
+static int reftable_index_record_cmp(const void *_a, const void *_b)
+{
+	const struct reftable_index_record *a = _a;
+	const struct reftable_index_record *b = _b;
+	return strbuf_cmp(&a->last_key, &b->last_key);
+}
+
 static void reftable_index_record_print(const void *rec, int hash_size)
 {
 	const struct reftable_index_record *idx = rec;
@@ -1094,6 +1145,7 @@ static struct reftable_record_vtable reftable_index_record_vtable = {
 	.release = &reftable_index_record_release,
 	.is_deletion = &not_a_deletion,
 	.equal = &reftable_index_record_equal,
+	.cmp = &reftable_index_record_cmp,
 	.print = &reftable_index_record_print,
 };
 
@@ -1147,6 +1199,14 @@ int reftable_record_is_deletion(struct reftable_record *rec)
 		reftable_record_data(rec));
 }
 
+int reftable_record_cmp(struct reftable_record *a, struct reftable_record *b)
+{
+	if (a->type != b->type)
+		BUG("cannot compare reftable records of different type");
+	return reftable_record_vtable(a)->cmp(
+		reftable_record_data(a), reftable_record_data(b));
+}
+
 int reftable_record_equal(struct reftable_record *a, struct reftable_record *b, int hash_size)
 {
 	if (a->type != b->type)
diff --git a/reftable/record.h b/reftable/record.h
index fd80cd451d..0d96fbfd1b 100644
--- a/reftable/record.h
+++ b/reftable/record.h
@@ -62,6 +62,12 @@ struct reftable_record_vtable {
 	/* Are two records equal? This assumes they have the same type. Returns 0 for non-equal. */
 	int (*equal)(const void *a, const void *b, int hash_size);
 
+	/*
+	 * Compare keys of two records with each other. The records must have
+	 * the same type.
+	 */
+	int (*cmp)(const void *a, const void *b);
+
 	/* Print on stdout, for debugging. */
 	void (*print)(const void *rec, int hash_size);
 };
@@ -114,6 +120,7 @@ struct reftable_record {
 };
 
 /* see struct record_vtable */
+int reftable_record_cmp(struct reftable_record *a, struct reftable_record *b);
 int reftable_record_equal(struct reftable_record *a, struct reftable_record *b, int hash_size);
 void reftable_record_print(struct reftable_record *rec, int hash_size);
 void reftable_record_key(struct reftable_record *rec, struct strbuf *dest);